Conventional turbine engine auxiliary power units are well known in the aerospace technologies. These units are used to provide power to the aircraft either on the ground or in flight, or both. This power may be provided in the form of one or more of electrical power, hydraulic power, pressurized air, or another form, according to the requirements of the aircraft in which the APU is installed. Unfortunately, starting of an APU may require from many seconds to as much as a few minutes. During this starting time power from the APU is, of course, not available to the aircraft. As a result, some essential aircraft systems may not be operated during starting of the APU. Also, if the aircraft is above a determined altitude, it is not possible to start the APU because of low ambient pressure. An aircraft with only an APU may require some other system, such as an EPU or a ram air turbine, to provide power to the aircraft until the APU can be started.
Similarly, EPU's are known which employ a hydrazine decomposition chamber, for example, or a jet fuel combustor to provide a flow of high temperature pressurized motive gas to a turbine. The turbine is employed to drive a hydraulic pump or electric generator, for example. The EPU is employed to provide hydraulic or electric power (or both) on a relatively short term basis after a failure of an essential system associated with the aircraft main engines. This emergency power supply allows continuation of controlled aircraft flight for a limited time while the aircraft is brought to a landing or to an altitude low enough to allow starting of the aircraft APU.
The development of unstable aircraft has in particular increased the necessity for providing a rapidly available source of emergency power. Upon a failure of the main hydraulic pump, or main generator, or of the aircraft propulsion engine driving these devices, the aircraft cannot be maintained in controlled flight. Without hydraulic power to move aircraft control surfaces, or electrical power for flight control computers, the unstable aircraft is uncontrollable. Thus, these aircraft must have a source of emergency power which is available almost immediately after the failure of a flight control related power system. Unfortunately, the conventional technology for this purpose employs hydrazine fuel and a decomposition chamber containing a catalytic reaction bed. When such an EPU is operated, even for a short time, the toxic hydrazine must be flushed from the aircraft system using neutralizing chemicals, and the decomposition chamber must be replaced.
As a result, a transient in the aircraft systems, for example, which does not represent a genuine emergency, but which exercises the EPU can be very expensive. Also, this type of unnecessary exercise of the EPU may compromise the availability of the EPU for operation were a genuine emergency to occur. As a result, EPU's which employ hydrazine and a decomposition chamber are widely recognized as an unsatisfactory solution to the need for an emergency power supply aboard aircraft.
Additional shortcomings of conventional technology which provides both an APU and a separate EPU are that the weight, size, fuel consumption, complexity, cost and maintenance requirements of the aircraft are all increased while the performance of the aircraft is decreased.